Category: PlaidCTF (2013)

ropasaurusrex: a primer on return-oriented programming

One of the worst feelings when playing a capture-the-flag challenge is the hindsight problem. You spend a few hours on a level—nothing like the amount of time I spent on cnot, not by a fraction—and realize that it was actually pretty easy. But also a brainfuck. That’s what ROP’s all about, after all!

Anyway, even though I spent a lot of time working on the wrong solution (specifically, I didn’t think to bypass ASLR for quite awhile), the process we took of completing the level first without, then with ASLR, is actually a good way to show it, so I’ll take the same route on this post.

Before I say anything else, I have to thank HikingPete for being my wingman on this one. Thanks to him, we solved this puzzle much more quickly and, for a short time, were in 3rd place worldwide!

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Epic “cnot” Writeup (highest value level from PlaidCTF)

When I was at Shmoocon, I saw a talk about how to write an effective capture-the-flag contest. One of their suggestions was to have a tar-pit challenge that would waste all the time of the best player, by giving him a complicated challenge he won’t be able to resist. In my opinion, in PlaidCTF, I suspected that “cnot” was that challenge. And I was the sucker, even though I knew it all the way…

(It turns out, after reviewing writeups of other challenges, that most of the challenges were like this; even so, I’m proud to have been sucked in!)

If you want a writeup where you can learn something, I plan to post a writeup for “Ropasaurus” in the next day or two. If you want a writeup about me being tortured as I fought through inconceivable horrors to finish a level and capture the bloody flag, read on! This level wasn’t a lot of learning, just brute-force persistence.

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